Embryonic development
Embryonic development also embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of , whereas the terms and describe later stages. Embryonic development starts with the of the (ovum) by a cell, (spermatozoon). Once fertilized, the ovum is referred to as a , a single cell. The zygote undergoes s with no significant growth (a process known as ) and , leading to development of a multicellular embryo. Although embryogenesis occurs in both and , this article addresses the common features among different animals, with some emphasis on the embryonic development of and . Fertilization and the zygote The egg cell is generally asymmetric, having an " " (future and ) and a " " (future ). It is covered with protective envelopes, with different layers. The first envelope – the one in contact with the of the egg – is made of and is known as the ( in ). Different show different cellular and acellular envelopes englobing the vitelline membrane. (also known as 'conception', 'fecundation' and 'syngamy') is the fusion of to produce a new organism. In animals, the process involves a fusing with an , which eventually leads to the development of an . Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation. The fertilized egg cell is known as the . To prevent more than one sperm fertilizing the egg, , fast block and slow block to polyspermy are used. Fast block, the membrane potential rapidly depolarizing and then returning to normal, happens immediately after an egg is fertilized by a single sperm. Slow block begins the first few seconds after fertilization and is when the release of calcium causes the cortical reaction, various enzymes releasing from cortical granules in the eggs plasma membrane, to expand and harden the outside membrane, preventing more sperm from entering. Cleavage and morula Cell division with no significant growth, producing a cluster of cells that is the same size as the original zygote, is called . At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the . The different cells derived from cleavage, up to the , are called s. Depending mostly on the amount of in the egg, the can be (total) or (partial). Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via the or , such as might be secreted from a . On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk (i.e. birds and reptiles). Because cleavage is impeded in the , there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, and the second cleavage is perpendicular to the first. From here the spatial arrangement of s can follow various patterns, due to different planes of cleavage, in various organisms: The end of cleavage is known as and coincides with the onset of zygotic . In amniotes, the cells of the are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, the , which does not contribute to the formation of the embryo proper, and an , from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a , called the . The inner cell mass remains in contact, however, with the trophoblast at one pole of the ovum; this is named the , since it indicates the location where the future embryo will develop. Formation of the blastula After the 7th cleavage has produced 128 s, the embryo is called a . The blastula is usually a spherical layer of cells (the ) surrounding a fluid-filled or yolk-filled cavity (the ). Mammals at this stage form a structure called the , characterized by an inner cell mass that is distinct from the surrounding blastula. The blastocyst must not be confused with the blastula; even though they are similar in structure, their cells have different fates. In the mouse, primordial s arise from a layer of cells in the inner cell mass of the (the ) as a result of extensive -wide reprogramming. Reprogramming involves global facilitated by the DNA pathway as well as reorganization, and results in cellular . Before , the cells of the trophoblast become differentiated into two strata: The outer stratum forms a (i.e., a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells), termed the , while the inner layer, the or "Layer of Langhans", consists of well-defined cells. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of the and play an important part in the development of the . On the deep surface of the inner cell mass, a layer of flattened cells, called the , is differentiated and quickly assumes the form of a small sac, called the . Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the is gradually developed. The floor of this cavity is formed by the , which is composed of a layer of s, the embryonic ectoderm, derived from the inner cell mass and lying in apposition with the endoderm. Formation of the germ layers The becomes oval and then pear-shaped, the wider end being directed forward. Near the narrow, posterior end, an opaque streak, called the , makes its appearance and extends along the middle of the disk for about one-half of its length; at the anterior end of the streak there is a knob-like thickening termed the or knot, (known as Hensen's knot in birds). A shallow groove, the , appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, the , with the . The primitive streak is produced by a thickening of the axial part of the ectoderm, the cells of which multiply, grow downward, and blend with those of the subjacent endoderm. From the sides of the primitive streak a third layer of cells, the , extends laterally between the ectoderm and endoderm; the end of the primitive streak forms the . The blastoderm now consists of three layers, named from without inward: ectoderm, mesoderm, and endoderm; each has distinctive characteristics and gives rise to certain tissues of the body. For many mammals, it is sometime during formation of the germ layers that of the embryo in the of the mother occurs. Formation of the gastrula During gastrulation cells migrate to the interior of the blastula, subsequently forming two (in animals) or three ( ) s. The embryo during this process is called a . The germ layers are referred to as the ectoderm, mesoderm and endoderm. In diploblastic animals only the ectoderm and the endoderm are present. * Among different animals, different combinations of the following processes occur to place the cells in the interior of the embryo: ** – expansion of one cell sheet over other cells ** Ingression – migration of individual cells into the embryo (cells move with ) ** – infolding of cell sheet into embryo, forming the , , and . ** Delamination – splitting or migration of one sheet into two sheets ** Involution – inturning of cell sheet over the basal surface of an outer layer ** Polar proliferation – Cells at the polar ends of the blastula/gastrula proliferate, mostly at the animal pole. * Other major changes during gastrulation: ** Heavy using embryonic genes; up to this point the s used were maternal (stored in the unfertilized egg). ** Cells start major processes, losing their iality. In most animals, a blastopore is formed at the point where cells are entering the embryo. Two major groups of animals can be distinguished according to the blastopore's fate. In s the anus forms from the blastopore, while in s it develops into the mouth. See for more information. Formation of the early nervous system – neural groove, tube and notochord In front of the primitive streak, two longitudinal ridges, caused by a folding up of the ectoderm, make their appearance, one on either side of the middle line formed by the streak. These are named the ; they commence some little distance behind the end of the , where they are continuous with each other, and from there gradually extend backward, one on either side of the anterior end of the primitive streak. Between these folds is a shallow groove, the . The groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, the or canal, the ectodermal wall of which forms the rudiment of the nervous system. After the coalescence of the neural folds over the anterior end of the primitive streak, the blastopore no longer opens on the surface but into the closed canal of the neural tube, and thus a transitory communication, the , is established between the neural tube and the primitive . The coalescence of the neural folds occurs first in the region of the , and from there extends forward and backward; toward the end of the third week, the front opening ( ) of the tube finally closes at the anterior end of the future , and forms a recess that is in contact, for a time, with the overlying ectoderm; the hinder part of the neural groove presents for a time a , and to this expanded portion the term has been applied. Before the neural groove is closed, a ridge of ectodermal cells appears along the prominent margin of each neural fold; this is termed the or ganglion ridge, and from it the and and the ganglia of the are developed. By the upward growth of the mesoderm, the neural tube is ultimately separated from the overlying ectoderm. The end of the neural groove exhibits several dilatations that, when the tube is shut, assume the form of three vesicles; these constitute the three primary , and correspond, respectively, to the future 'fore-brain' ( ), ' ' (mesencephalon), and 'hind-brain' ( ) (Fig. 18). The walls of the vesicles are developed into the nervous tissue and neuroglia of the brain, and their cavities are modified to form its ventricles. The remainder of the tube forms the (medulla spinalis); from its ectodermal wall the nervous and neuroglial elements of the spinal cord are developed, while the cavity persists as the . Formation of the early septum The extension of the mesoderm takes place throughout the whole of the embryonic and extra-embryonic areas of the ovum, except in certain regions. One of these is seen immediately in front of the neural tube. Here the mesoderm extends forward in the form of two es, which meet in the middle line so as to enclose behind them an area that is devoid of mesoderm. Over this area, the ectoderm and endoderm come into direct contact with each other and constitute a thin membrane, the , which forms a septum between the primitive mouth and . Early formation of the heart and other primitive structures In front of the buccopharyngeal area, where the lateral crescents of mesoderm fuse in the middle line, the is afterward developed, and this region is therefore designated the pericardial area. A second region where the mesoderm is absent, at least for a time, is that immediately in front of the pericardial area. This is termed the , and is the region where the is developed; in humans, however, it appears that a proamnion is never formed. A third region is at the hind end of the embryo, where the ectoderm and endoderm come into apposition and form the cloacal membrane. Somitogenesis Somitogenesis is the process by which s (primitive segments) are produced. These segmented tissue blocks differentiate into skeletal muscle, vertebrae, and dermis of all vertebrates. Somitogenesis begins with the formation of (whorls of concentric mesoderm) marking the future somites in the presomitic mesoderm (unsegmented paraxial). The presomitic mesoderm gives rise to successive pairs of somites, identical in appearance that differentiate into the same cell types but the structures formed by the cells vary depending upon the anteroposterior (e.g., the vertebrae have ribs, the vertebrae do not). Somites have unique positional values along this axis and it is thought that these are specified by the s. Toward the end of the second week after fertilization, segmentation of the begins, and it is converted into a series of well-defined, more or less cubical masses, also known as the somites, which occupy the entire length of the trunk on either side of the middle line from the region of the head. Each segment contains a central cavity (known as a ), which, however, is soon filled with angular and spindle-shape cells. The somites lie immediately under the ectoderm on the lateral aspect of the neural tube and , and are connected to the by the . Those of the trunk may be arranged in the following groups, viz.: 8, 12, 5, 5, and from 5 to 8. Those of the occipital region of the head are usually described as being four in number. In mammals, somites of the head can be recognized only in the occipital region, but a study of the lower vertebrates leads to the belief that they are present also in the anterior part of the head and that, altogether, nine segments are represented in the cephalic region. Organogenesis At some point after the different germ layers are defined, begins. The first stage in s is called , where the folds forming the neural tube (see above). Other common organs or structures that arise at this time include the and somites (also above), but from now on embryogenesis follows no common pattern among the different taxa of the . In most animals organogenesis, along with , results in a . The hatching of the larva, which must then undergo , marks the end of embryonic development. References Category:Life